Ion implanters are commonly used in the manufacture of semiconductor and metallic products for implanting ions into semiconductor or metallic substrates to change the conductivity of the material in such substrates or in predefined regions thereof. Ion implanters generally comprise an ion beam generator for generating a beam of ions, and means for selecting a particular species of ions in the ion beam and for directing the ion beam through a vacuum chamber onto a target substrate supported on a substrate holder.
In typical prior art implanters, for sequentially processing single wafers, such as are disclosed in U.S. Pat. Nos. 4,929,840, 4,975,586 and 5,003,183, the substrate holder is mounted for rotation so as to rotate the holder in front of the ion beam, with the substrate holder being rotatable about a first axis extending substantially along a wafer diameter in the plane defined by the wafer surface, and about a second axis perpendicular to the first axis and extending through the centre of the wafer. The drive systems providing for rotation of the wafer support are described as being operable independently of one another and include stepper motors mounted outside the vacuum chamber.
The motor for rotating the wafer holder about the wafer axis is coupled to the wafer holder via two relatively lengthy drive shafts coupled by a belt transmission, the final of the two drive shafts being coupled to the wafer holder by a bevel gear arrangement.
In U.S. Pat. No. 5,229,615, there is disclosed a single wafer ion implanter in which a substrate holder, or platen assembly, is mounted for rotation about the wafer axis on an arm and is coupled to a motor by a chain drive system. The arm has a housing containing the motor and is arranged also to enclose the chain drive system. The resulting arm is bulky in cross-section and relatively heavy.